1. Field of Invention
The present invention relates to a die-lateral extruding method and apparatus.
2. Description of Related Art
Forging of aluminum has various advantages over forging of steel such as scale does not form; furthermore, the forging temperature is low. Forging of aluminum is therefore requested to apply to parts having more complex shape than the steel die-forgings. The producers have recognized such requests.
The history of forging of aluminum has started with the production of aircraft parts where weight reduction was essential. Necessarily precision-forging of parts having complex shape has been achieved by many processes. Many processes have been developed to achieve the precision-forging of parts of complex shape; however, productivity was low. Along with the trend to high demand for aluminum forgings in the automobile industries, cost reduction in such processes has become necessary so that they can replace conventional forging methods.
Since the conventional forging of aluminum has disadvantages of low material yield and productivity, a new plastic working method of aluminum free of such disadvantages must be provided.
Meanwhile, in the production of axially symmetrical forgings, metal bar stock, which may have various shapes, is compression-worked in the axial direction. The metal flow thus formed is virtually ideal and the cavity of the finishing metal die can be filled with the metal. Contrary to this, in the production of forgings having axially non-symmetrical shape, the metal bar stock is subjected to preliminary shaping in multiple steps with rough-forming metal dies. The preliminarily shaped forgings are then subjected to repeated processes to produce the final shape. The metal bar stock is therefore gradually shaped close to the final shape.
The preliminarily shaped article obtained in the production of axially non-symmetrical forgings has the volume, orientation and distribution of material which are appropriate for the subsequent forging to obtain the axially non-symmetrical shape. The hot-forging process for obtaining a preliminarily shaped article is carried out in multisteps. The forging is therefore not completed in a one-heat process.
Particularly in the case of a product having a high axial non-symmetry, not only is the forging process in multisteps, but also a trimming step for removing burrs from the forged material must be inserted between every forging steps; consequently, the material yield and productivity are seriously impaired. In addition, energy consumption for reheating increases. When the multisteps are omitted and the cavity of a metal die is filled with material at an extremely high pressure, internal and external defects such as indentation and cracks are formed, so that defect-free products cannot be obtained.
Meanwhile, the extrusion is used for working the metallic materials. By the extrusion, a high average pressure is applied to a work piece, so that it can be shaped at once into a product having a complicated cross sectional shape. Since the plastic workability property of aluminum is excellent, almost all cross sectional shapes, such as round, rectangular and groove as well as a pipe can be shaped by extrusion of aluminum.
A conventional extrusion method is described with reference to the production of pipe material. Since a billet used for the production of a pipe has a greater diameter than that of the product, the working load and hence the capacity of the working apparatus become high. In addition, since the wall thickness of a pipe is determined by the distance between the inner diameter of a die and the outer diameter of mandrel, the wall thickness locally greatly varies, which is one of the disadvantages of the extrusion method.
In order to overcome the disadvantages of the extrusion method, a Lafro method is proposed (c.f., U.S. Pat. No. 3,263,468 issued to D. W. Rowell, and "Wire Industry" 1976, p 903). In the Lafro method, a metal bar stock is extruded by a plunger, and is subjected to working by a mandrel which converts the flow vector of metal-bar stock into a radial direction. The so worked metal is then subjected to working by a die which again converts its flow vector so as to impart the metal the pipe shape. However, it is only the pipe that can be produced by the Lafro method.
There are reports related to the Lafro method in Study on Radial Flow Extrusion in Aluminum (by Y. Matsuura et al. Imoken Report No. 41, 1985 p49-57) and Study on Extrusion of Large Pipes by Using Smaller Billets (T. Yamada et al., J. of Jn Metal Soc. Vol. 46, No. 10, 1982, p1023-1029).